doc: python support matrix

Fixes: https://pagure.io/koji/issue/356

docs/source/index.rst

@@ -49,6 +49,7 @@ Contents
     content_generator_metadata
     configuring_jenkins
     utils
+    supported_platforms
 
 HowTos
 ======

docs/source/supported_platforms.rst

@@ -0,0 +1,13 @@
+Supported Platforms
+===================
+
+We're now supporting Linux systems which have at least python 2.7 for
+builders and 3.6 for other components. These versions are minimal (so,
+everywhere where is 2.7 support it means 2.7+ *and* 3.0+. Currently it
+involves all active Fedoras and RHEL/CentOS 7+.
+
++-----------+-----+-----+---------+-------+-----+-----+
+| Component | Hub | Web | Builder | Utils | Lib | CLI |
++===========+=====+=====+=========+=======+=====+=====+
+| Python    | 3.6 | 3.6 | 2.7     | 3.6   | 2.7 | 2.7 |
++-----------+-----+-----+---------+-------+-----+-----+

docs/source/writing_koji_code.rst

@@ -14,6 +14,11 @@ to Koji should leave this page knowing where to begin and have enough
 understanding of Koji's architecture to be able to estimate how much
 work is still ahead of them.
 
+Koji is written to support a variety of platforms and python versions,
+with some portions of Koji supported over a larger set than others.
+When making changes, please be considerate of those
+:doc:`supported_platforms`.
+
 Task Flow
 =========
 
@@ -689,6 +694,9 @@ You will need to install the packages below to run the check.
 Release process
 ===============
 
+Supported OS
+------------
+
 Merging PRs
 -----------